The present disclosure relates generally to photodisruption induced by a pulsed laser beam and the location of the photodisruption so as to treat a material, such as a tissue of an eye. Although specific reference is made to locating target site(s) for photodisruption and cutting tissue for surgery such as eye surgery, embodiments as described herein can be used in many ways with many materials to treat one or more of many materials, such as cutting of optically transparent materials.
Cutting of materials can be done mechanically with chisels, knives, scalpels and other tools such as surgical tools. However, prior methods and apparatus of cutting can be less than desirable and provide less than ideal results in at least some instances. For example, at least some prior methods and apparatus for cutting materials such as tissue may provide a somewhat rougher surface than would be ideal. Pulsed lasers can be used to cut one or more of many materials and have been used for laser surgery to cut tissue.
Examples of surgical tissue cutting include cutting the cornea and crystalline lens of the eye. The lens of the eye can be cut to correct a defect of the lens, for example to remove a cataract, and the tissues of the eye can be cut to access the lens. For example the cornea can be cut to access the cataractous lens. The cornea can be cut in order to correct a refractive error of the eye, for example with laser assisted in situ keratomileusis (hereinafter “LASIK”).
Many patients may have less than ideal optics of the eye. At least some patients may have refractive error of the eye that can be corrected with spectacles and contact lenses, for example. However, patients may have an irregularity of the cornea of eye such as irregular astigmatism or corneal scarring from example. In at least some instances, the irregularity of the eye may not be easily corrected with prior methods and apparatus. Prior approaches to treating the diseased cornea have included keratoplasty, such as penetrating keratoplasty (hereinafter “PK”), for example. The prior keratoplasty procedures can result in less than ideal patient outcomes in at least some instances. For example, patients may have less than ideal visual acuity after keratoplasty procedures. In at least some instances, such less than ideal visual acuity may be caused than less than ideal positioning and location of tissue cuts.
Prior short pulse laser systems have been used to cut tissue, and have been used to treat many patients. However, the prior short pulse systems may provide less than ideal results in at least some instances. For example, the alignment of the eye with the laser surgery system can be less than ideal in at least some instances, such as when refractive treatment of the cornea of the eye is combined with a treatment of the lens of the eye such as removal of the cortex and nucleus from the eye. In another example, the laser eye surgery system may not properly take into account the different indices of refraction of the eye anatomy in at least some instances, which may affect the positioning of tissue cuts in at least some instances.
In order to more accurately treat the eye, prior methods and apparatus have combined optical measurement systems such as tomography systems. However, the accuracy of such prior measurement devices can be less than ideal in at least some instances. For example, to determine the physical location of a structure, the prior devices may rely on an assumed index of refraction which can vary from the actual index of refraction of the particular eye of an individual being treated. Further, at least some prior devices may rely on an assumed average value of the index of refraction for tissues that have a varying index of refraction such as tissue of the lens. The amount of variation of the index of refraction within an individual may vary more, or less, than normative values for a population, potentially making assumed values less accurate in at least some instances. In at least some instances, the treatment beam may comprise different wavelengths than the measurement beam, potentially further compounding the errors in the measurements in at least some instances.
The decreased accuracy of the prior methods an apparatus may limit, in at least some respects, the treatment of the prior methods and apparatus. For example, variability of the index of refraction may result in variability of the depth at which tissue is incised, thereby potentially decreasing the accuracy of the prior surgical procedures and potentially limiting the use of lasers to incise tissue near sensitive.
In light of the above, it would be desirable to provide improved methods and apparatus that overcome at least some of the above limitations of the above prior systems and methods. Ideally, these improved systems and methods will provide in situ measurement of the index of refraction of optically transmissive materials, provide improved measurement of the location of structures within the optically transmissive materials, and provide improved treatment with more accurate focus of laser beams within the material.